The present invention relates to a toner for developing electrostatic images used in image forming methods, such as electrophotography, electrostatic recording or electrostatic printing, particularly a toner suitable for hot roller fixation.
Hitherto, a large number of electrophotographic processes have been known, inclusive of those disclosed in U.S. Pat. Nos. 2,297,691; 3,666,363; and 4,071,361. In these processes, in general, an electrostatic latent image is formed on a photosensitive member comprising a photoconductive material by various means, then the latent image is developed with a toner, and the resultant toner image is, after being transferred onto a transfer material such as paper etc., as desired, fixed by heating, pressing, or heating and pressing, or with solvent vapor to obtain a copy or print carrying a fixed toner image.
As for the step of fixing the toner image onto a sheet material such as paper which is the final step in the above process, various methods and apparatus have been developed, of which the most popular one is a heating and pressing fixation system using hot rollers.
In the heating and pressing system, a sheet carrying a toner image to be fixed (hereinafter called xe2x80x9cfixation sheetxe2x80x9d) is passed through hot rollers, while a surface of a hot roller having a releasability with the toner is caused to contact the toner image surface of the fixation sheet under pressure, to fix the toner image. In this method, as the hot roller surface and the toner image on the fixation sheet contact each other under a pressure, a very good heat efficiency is attained for melt-fixing the toner image onto the fixation sheet to afford quick fixation.
It is however a current state that different toners are used for different models of copying machines and printers. This is primarily because the different models adopt different fixing speeds and fixing temperatures. More specifically, in the fixing step, a hot roller surface and a toner image contact each other in a melted state and under a pressure, so that a part of the toner is transferred and attached to the fixing roller surface and then re-transferred to a subsequent fixation sheet to soil the fixation sheet. This is called an offset phenomenon and is remarkably affected by the fixing speed and temperature. Generally, the fixing roller surface temperature is set to be low in case of a slow fixing speed and set to be high in case of a fast fixing speed. This is because a constant heat quantity is supplied to the toner image for fixation thereof regardless of a difference in fixing speed.
However, the toner on a fixation sheet is deposited in several layers, so that there is liable to occur a large temperature difference between a toner layer contacting the heating roller and a lowermost toner layer particularly in a hot-fixation system using a high heating roller temperature. As a result, a topmost toner layer is liable to cause an offset phenomenon in case of a high heating roller temperature, while a low-temperature offset is liable to occur because of insufficient melting of the lowermost toner layer in case of a low heating roller temperature.
In order to solve the above problem, it has been generally practiced to increase the fixing pressure in case of a fast fixing speed in order to promote the anchoring of the toner onto the fixation sheet. According to this method, the heating roller temperature can be somewhat lowered and it is possible to obviate a high-temperature offset phenomenon of an uppermost toner layer. However, as a very high shearing force is applied to the toner layer, there are liable to be caused several difficulties, such as a winding offset that the fixation sheet winds about the fixing roller, the appearance of a trace in the fixed image of a separating member for separating the fixation sheet from the fixing roller, and inferior copied images, such as resolution failure of line images and toner scattering, due to a high pressure.
Accordingly, in a high-speed fixing system, a toner having a lower melt viscosity is generally used than in the case of low speed fixation, so as to lower the heating roller temperature and fixing pressure, thereby effecting the fixation while obviating the high-temperature offset and winding offset. However, in the case of using such a toner having a low melt viscosity in low speed fixation, an offset phenomenon is liable to be caused because of the low viscosity.
Accordingly, there has been desired a toner which shows a wide fixable temperature range and an excellent anti-offset characteristic and is applicable from a low speed apparatus to a high speed apparatus.
On the other hand, in recent years, there have been also desired high-quality copy or print images in accordance with the use of digitalized copying machines and fine toner particles.
More specifically, it has been desired to obtain a photographic image accompanied with characters, so that the character images are clear while the photographic image is excellent in density gradation faithful to the original. Generally, in a copy of a photographic image accompanied with characters, if the line density is increased so as to provide clear character images, not only the density gradation characteristic of the photograph image is impaired, but also the halftone part thereof are roughened.
Further, resolution failure (collapsion) of line images and scattering are liable to be caused at the time of fixation as described above, so that the image qualities of the resultant copy images are rather liable to be deteriorated.
Further, in case where the line image density is increased, because of an increased toner coverage, a thick toner image is pushed against a photosensitive member to be attached to the photosensitive member in the toner transfer step, so that a so-called transfer failure (or a hollow image), i.e., a partial lack toner image (line images in this case), in the transferred image, is liable to be caused, thereby providing poor quality of copy images. On the other hand, in case where the gradation characteristic of a photographic image is intended to be improved, the density of characters or line images are liable to be lowered, thus providing unclear images.
In recent years, there has been obtained some improvement in density gradation characteristic by a system including image density readout and digital conversion. However, a further improvement has been desired.
Regarding density gradation characteristic, it is impossible to obtain a linear relationship between a developing potential (difference between a photosensitive member potential and a developer-carrying member potential) and a resultant (copy) image density. More specifically, as shown in FIG. 1, a characteristic curve (e.g., a solid curve representing a case of providing a maximum intensity of 1.4) becomes downwardly convex at a low developing potential and upwardly convex at a high developing potential. Accordingly, in a halftone region, a slight change in developing potential leads to a remarkable change in image density. This provides a complexity in obtaining a satisfactory density gradation characteristic.
Generally, copied images appear clearer because of an edge effect so that clear line images can be retained in case where a maximum density of ca. 1.30 is attained at a solid image part which is less affected by the edge effect.
In case of a photographic image, however, the maximum density of a photograph appears less at a glance because of its surface gloss but actually amounts to a very high level of 1.90-2.00. Accordingly, in a copy of a photographic image, even if the surface gloss is suppressed, a solid part image density of ca. 1.4-1.5 is required since a density increase due to the edge effect cannot be excepted because of a large image area.
Accordingly, in providing a copy of a photographic image accompanied with characters, it becomes very important to obtain a developing potential-image density relationship which is close to the first order (linear) one and also a maximum image density of 1.4-1.5.
Further, the density gradation characteristic is liable to be remarkably affected by the saturation charge and the charging speed of a developer used. In case where the saturation charge is appropriate for the developing conditions, a developer showing a slow charging speed provides a low maximum image density, thus generally thin and blurred images in the initial stage of copying. In this case, however, non-problematic images can be obtained if the maximum image density is ca. 1.3, as described above, thus being able to obviate an adverse effect of the slow chargeability. Even in case of the slow charging speed, the initial copy image density is increased if the saturation charge is increased. However, on continuation of copying, the charge of the developer is gradually increased to finally exceed an appropriate charge for development, thereby resulting in a lower copy image density. Also in this case, no problem occurs in line images if the maximum image density is ca. 1.3.
From the above, it is understood that a photographic image is more remarkably affected by the saturation charge and the charging speed of a developer than a line image.
The use of a smaller particle size toner can increase the resolution and clearness of an image but is also liable to be accompanied with various difficulties.
First, a smaller particle size toner is liable to impair the fixability of a halftone image. This is particularly noticeable in high-speed fixation. This is because the toner coverage in a halftone part is little and a portion of toner transferred to a concavity of a fixation sheet receives only a small quantity of heat and the pressure applied thereto is also suppressed because of the convexity of the fixation sheet. A portion of toner transferred onto the convexity of the fixation sheet in a halftone part receives a much larger shearing force per toner particle because of a small toner layer thickness compared with that in a solid image part, thus being liable to cause offset or result in copy images of a lower image quality.
Fog is another problem. If the toner particle size is reduced, the surface area of a unit weight of toner is increased, so that the charge distribution thereof is liable to be broadened to cause fog. As the toner surface area is increased per unit weight thereof, the toner chargeability is liable to be affected by a change in environmental conditions.
If the toner particle size is reduced, the dispersion state of a polar material and a colorant is liable to affect the toner chargeability.
When such a small particle size toner is applied to a high-speed copying machine, the toner is liable to be excessively charged to cause fog and a density decrease, particularly in a low-humidity environment.
Further, in connection with a trend of providing a copying machine with a multiplicity of functions, such as a superposed multi-color copying of erasing a part of an image as by exposure and inserting another image into the erased part, or frame erasure of erasing a frame part on a copying sheet, fog of a small particle size is liable to remain in such a part to be erased into white.
When an image is erased by providing a potential of a polarity opposite to that of a latent image potential with respect to a development reference potential as by irradiation with intense light from LED, a fuse lamp, etc., the erased part is liable to cause fog.
Japanese Laid-Open Patent Application (JP-A) 59-129863 and JP-A 3-50561 have proposed the use of a polyester resin and an acid-modified polyolefin. According to the proposal, maleic anhydride is added to polyolefin synthesized in advance. In case where an acid anhydride is added, the polarity obtained thereby is very weak, so that it is impossible to break an association of polymer OH groups. Accordingly, in an initial stage of copying, the charging speed is fast to provide a high charge because of associations of polymer carboxylic groups. In this instance, the toner quantity used for development is large to provide high image density copies. However, as many associations of polymer OH groups are present, the saturation charge is gradually reduced so that the copy image density is gradually lowered correspondingly.
Maleic anhydride used in the above proposals react with water to open its ring but, even in such a case, the associatability the resultant carboxylic group is lowered because of an adjacent carboxylic group. Further, maleic acid is not always attached to molecular chain terminals. Accordingly, when maleic acid is attached to a middle of a molecular chain, this is identical to branching of the molecule chain. Further, according to the proposed method utilizing a post addition reaction, it is very difficult to add one maleic acid to each molecular chain. Accordingly, plural carboxyl groups may be introduced into one molecule chain, thereby resulting in a lower associatability. In this case, the charging speed and the environmental stability are liable to be lowered.
U.S. Pat. No. 4,883,736, JP-A 4-97162 and JP-A 4-204543 disclose methods of using aliphatic alcohols. In these methods, however, no carboxylic group association is formed, so that the resultant charging speed is slow, whereby the density gradation characteristic of copy images is not stabilized in a digital copying machine.
A generic object of the present invention is to provide a toner for developing electrostatic images having solved the above-mentioned problems.
A more specific object of the present invention is to provide a toner for developing electrostatic images showing an excellent anti-offset characteristic without impairing the fixability from a low fixing speed to a high fixing speed.
Another object of the present invention is to provide a toner for developing electrostatic images, even in a small particle size, capable of showing a good fixability at a halftone part and providing copy images of good image quality from low to high process speed and fixing speed.
Another object of the present invention is to provide a tone for developing electrostatic images capable of providing high-density copy images free from fog from a low to a high process speed.
Another object of the present invention is to provide a toner for developing electrostatic images capable of providing good images in a low-humidity environment and also in a high-humidity environment without being affected by a change in environmental conditions.
Another object of the present invention is to provide a toner for developing electrostatic images applicable to wide variety of models of image forming apparatus.
Another object of the present invention is to provide a toner for developing electrostatic images having excellent durability and capable of providing copy images having a high image density and free from fog even in a long period of continuous image formation on a larger number of sheets.
Another object of the present invention is to provide copies of a photographic image with characters including clear character images and photographic images having a density gradation characteristic faithful to the original.
According to the present invention, there is provided a toner for developing an electrostatic image, comprising: a binder resin and a long-chain compound,
wherein the binder resin comprises a polyester resin having an acid value, and
the long-chain compound comprises a long-chain alkyl alcohol having an OH value of 10-120 mgKOH/g or a long-chain alkyl carboxylic acid having an acid value of 5-120 mgKOH/g and is contained so as to satisfy a condition of the following formula (1) or formula (2):
Formula (1)
acid value of binder resin +OH value of long-chain alkyl alcohol  greater than (xc2xc)xc3x97OH value of binder resin, or
Formula (2)
acid value of binder resin +acid value of long-chain alkyl carboxylic acid  greater than (xc2xc)xc3x97OH value of binder resin.
According to another aspect of the present invention, there is provided a toner for developing an electrostatic image, comprising: a binder resin and a long-chain compound,
wherein the binder resin comprises a vinyl resin having an acid value of 2.5-70 mgKOH/g, and
the long-chain compound comprises a long-chain-alkyl alcohol having an OH value of 10-120 mgKOH/g or a long-chain alkyl carboxylic acid having an acid value of 5-120 mgKOH/g and is contained so as to satisfy a condition of the following formula (1) or formula (2):
Formula (1)
acid value of binder resin +OH value of long-chain alkyl alcohol  greater than (xc2xc)xc3x97OH value of binder resin, or
Formula (2)
acid value of binder resin +acid value of long-chain alkyl carboxylic acid  greater than (xc2xc)xc3x97OH value of binder resin.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.